Apparatus for removing NOx from a gas

Abstract

An ammonia injection rate to an apparatus for catalytically removing nitrogen oxides from a combustion flue gas in the presence of ammonia, provided in a flue gas passage, is controlled in accordance with changes in load by a system comprising a mole ratio computing unit for computing a mole ratio signal from a gas temperature signal from a gas temperature dector, an inlet NO x detection signal from an inlet NO x detector, a gas flow rate detection signal from a gas flow rate detector, and a leaked NH 3 setting signal from a leaked NH 3 setting unit; a leaked NH 3 computing unit for computing a leaked NH 3 computing signal from the inlet NO x detection signal from the inlet NO x detector, an outlet NO x detection signal from an outlet NO x detector, and the mole ratio signal from the mole ratio computing unit; and a comparator for computing a leaked NH 3 deviation signal from the leaked NH 3 setting signal and the leaked NH 3 computing signal, the leaked NH 3 deviation signal being added to the mole ratio signal, thereby obtaining a mole ratio-correcting signal, and an ammonia injection rate being controlled by the mole ratio correcting signal.

What is claimed is: 
     
       1. An apparatus for removing NO x  from gases which comprises: an NO x  removal unit provided in a flue gas passage and having means to inject ammonia into said passage;   a gas temperature detector provided in fluid communication with said gas passage and upstream of said NO x  removal unit for generating a gas temperature detection signal;   an inlet NO x  detector provided in fluid communication with said gas passage and upstream of said NO x  removal unit for generating an inlet NO x  detection signal;   a gas flow rate detector provided in fluid communication with said gas passage for generating a gas flow rate detection signal;   a leaked NH 3  setting unit for generating a leaked NH 3  setting signal;   an outlet NO x  detector provided in said flue gas passage downstream of said NO x  removal unit for generating an outlet NO x  detection signal;   a mole ratio computing unit for computing a mole ratio signal from the gas temperature detection signal, the inlet NO x  detection signal, the gas flow rate detection signal and the leaked NH 3  setting signal;   means for transmitting each of the gas temperature detection signal from said gas temperature detector, the inlet NO x  detection signal from said inlet NO x  detector, the gas flow rate detection signal from said gas flow rate detector and the leaked NH 3  setting signal from said leaked NH 3  setting unit to said mole ratio computing unit;   a leaked NH 3  computing unit for computing a leaked NH 3  computing signal from the inlet NO x  detection signal, the outlet NO x  detection signal and the mole ratio signal;   means for transmitting each of the inlet NO x  detection signal from said inlet NO x  detector, the outlet NO x  detection signal from said outlet NO x  detector and the mole ratio computing unit to said leaked NH 3  computing unit;   a comparator for computing a leaked NH 3  deviation signal from the leaked NH 3  setting signal and the leaked NH 3  computing signal;   means for transmitting each of the leaked NH 3  setting signal from said leaked NH 3  setting unit and the leaked NH 3  computing signal from said leaked NH 3  computing unit to said comparator;   means for adding the leaked NH 3  deviation signal to the mole ratio signal, thereby obtaining a mole ratio-correcting signal;   means for transmitting each of the leaked NH 3  deviation signal from said comparator and the mole ratio signal from said mole ratio computing unit to said means for adding;   means for controlling an ammonia injection rate to said ammonia injection means by the mole ratio-correcting signal; and   means for transmitting the mole ratio-correcting signal from said means for adding to said means for controlling.   
     
     
       2. An apparatus according to claim 4, wherein the mole ratio computing unit comprises means for computing the mole ratio, M", according to the following equation:   M"=F(G, T, NO.sub.xi, S--NH.sub.3)     wherein:   G: flue gas flow rate in N m 3  /hr,   T: flue gas temperature in °C.,   NO xi  : inlet NO x  in ppm, and   S--NH 3  : leaked NH 3  in ppm.   
     
     
       3. An apparatus according to claim 4, wherein the leaked NH 3  computing unit comprises means for computing leaked NH 3  according to the following equation:   S--NH.sub.3 =F(M", NO.sub.xi, N.sub.xo)     wherein:   S--NH 3  : leaked NH 3  in ppm,   NO xi  : inlet NO x  in ppm,   M": NH 3  /NO x  mole ratio, and   NO xo  : outlet NO x  in ppm.   
     
     
       4. An apparatus according to claim 1, wherein said injection means comprises an ammonia injection pipe in said flue gas pasage downstream of said gas temperature detector and said inlet NO x  detector and upstream of said NO x  removal unit. 
     
     
       5. An apparatus according to claim 1, wherein said means for controlling the ammonia injection rate comprises a multiplier for generating a total NO x  amount signal from the gas flow rate detection signal and the inlet NO x  detection signal, and a multiplier for determining the ammonia injection rate from the mole ratio-correcting signal and the total NO x  amount signal. 
     
     
       6. An apparatus according to claim 1, wherein said gas temperature detector is provided in said flue gas passage. 
     
     
       7. An apparatus according to claim 6, wherein said inlet NO x  detector is provided in said flue gas passage. 
     
     
       8. An apparatus according to claim 7, wherein said outlet NO x  detector is provided in said flue gas pasage.